1. Field of the Invention
The invention relate to bar code readers, and more particularly to bar code readers of the kind where a beam of light, such as a laser beam, is caused to scan a bar code so that reflected light as a series of pulses can analysed and the bar code be decoded.
When a bar code is to be read, it may be oriented in an arbitrary direction. To read this bar code, the scanning pattern has to be designed such that, at least, one of its scanning lines passes over this bar code. The pattern, therefore, has to be effective over various directions and a multiple-directional scanning pattern is desirable.
In most multiple-directional scanners, a rotation mirror or an array of mirrors (polygon) is used to produce scanning lines. These lines are further separated and projected by auxiliary mirrors to form the multiple-directional scanning pattern.
2. Description of the Prior Art
A typical scanning scanner with a single rotating mirror has been described in U.S. Pat. No. 4,699,447. In this scanner, a set of mirrors surround this rotating mirror. The laser beam, incident on the rotating mirror, is directed to these mirrors. Reflecting from them, it scans the bar code and projects a multiple-directional pattern on it. The number of the scanning lines with various orientations depends on the number of these mirrors and their orientations.
Instead of a single mirror, a rotation polygon can also be used. In a typical scanner, described in U.S. Pat. No. 4,797,551, the laser is reflected by the facets of this polygon to a set of mirrors, surrounding this polygon. Reflected by them, a multiple-directional scanning pattern is projected onto the bar code.
However, the dimension and distribution of the scanning lines (scanning pattern), generated by the above-mentioned scanners, are changed at different reading distances. Therefore, the opportunity of reading bar codes may only be optimal over a restricted longitudinal working range. For instance, in a plane the scanning pattern may be star-like around a symmetrical axis, while in any other plane the scanning lines will be shifted away from this axis. In the area around this axis no bar code can be read.
Various scanning systems have been devised for use in producing line and frame scans for television operation, for example GB 1273176 and 2048606 and generally such systems use two motors, one for the line scan and one for the frame scan.
Systems for other purposes, in particular
GB 1 145 436 for recording information such as on a thermoplastic record medium;
GB 1 541 214 for heating a work piece such as in welding thin plastics film in making bags;
U.S. Pat. No. 4,838,632 for two dimensional digital film recording; and
U.S. Pat. No. 4,941,739 for provision of a mirror scanning system for use in a robotic manipulator,
generally also use two or more motors.
Finally, PCT application WO 92/09133 discloses a bar code scanner where a mirror is mounted on a spring platform and oscillates in a short arc about three axes so that a form of varying star pattern is produced.
Such a system is costly to make and difficult to control.